To get a trial license, go to the Free Trial page at https://www.autodesk.com/products/arnold/trial-intake
You have to click the Download button. That’s what generates the trial license and adds it to your Autodesk account. But you don’t need that download (it’s the Arnold SDK, not an Arnold plugin), so Cancel that download after it starts.
You can download the Arnold plugin of your choice from arnoldrenderer.com/download.
To use the trial license, install the Arnold plugin and then start the Arnold License Manager and sign in.